In general, operational amplifier circuits which utilize switched capacitors have an additional output voltage error which is separate from the offset voltage normally associated with an operational amplifier. Such additional contributions to the offset voltage result from both parasitic diffusion capacitance and gate overlap capacitance associated with switches used to switch the circuit's capacitors. Parasitic diffusion capacitance exists at every pn junction formed between a substrate and an implant in a conventional MOS transmission gate switch. Gate overlap capacitance occurs between every pn junction of a transmission switch and the gate in a conventional MOS transmission gate switch when the gate region slightly overlaps above the implanted region. Unfortunately, parasitic diffusion capacitance and gate overlap capacitance negatively affect both the AC and DC performance of an operational amplifier circuit. Typically, compensated switches having symmetrical structures can be used to reduce the DC component of output voltage error produced by the effects of gate overlap capacitance. The symmetrical nature of the compensated switch tends to cause charges which are injected through the parasitic gate overlap capacitances to nullify each other. However, compensated switches do not substantially reduce the AC component of output voltage error caused by either type of parasitic capacitance.
A partial solution to eliminate both AC and DC offset errors resulting from gate overlap and diffusion capacitance effects has been to use additional compensated switches in a symmetrical bridge switching structure. Although the use of compensated switches in a symmetrical bridge switching structure substantially eliminates the DC component of offset voltages due to gate overlap capacitance and the symmetrical structure of such circuits substantially eliminates undesirable effects of the parasitic capacitance on the AC precision, such circuits do not compensate for the DC parasitic diffusion component to the output voltage.